A growing number of metalloproteins of diverse function have in common the Fe-O-Fe group: hemerythrin is an oxygen carrier in several phylla; the diiron purple acid phosphatases are widely distributed in Nature; porcine uteroferrin may have dual phosphatase/iron transport roles; ribonucleotide reductase, which is ubiquitous, mediates the conversion of (oxy)ribonucleic acids to deoxyribonucleic acids -- control of this process may lead to new forms of cancer treatment, distinct from the intercalating cis-platinum drugs. While the active site of hemerythrin is well-characterized, the structure and mechanism of the others listed are not understood. In all cases the iron atoms are inequivalent. In order to elucidate structure-function relationships, it is essential to develop the currently meager chemistry if unsymmetrical coordinatively diiron systems. Recently we characterized the first unsymmetrical Mu-oxo diiron complex, [N5FeO-FeCl-3]Cl.2C-2H-5OH, where N5 is an unsymmetrical pentadentate benzimidazole-bearing ligand that is amenable to further functionalization. In particular this complex has magnetic properties very similar to met-hemerythrin (met-Hr). We propose developing further the distinctive chemistry of this system through: 1. Determining by 1H NMR, chemical shifts and relaxation times of the imidazole N-H protons for elucidation of comparable spectra for Hr, which has five histidine ligands, and for the identification of imidazole groups in other diiron proteins. 2. Ligand substitution at the tetrahedral Fe (phenolate for oxidized uteroferrin models). 3. Reduction to mixed valence N5Fe(II)OFe(III)X3 species (uteroferrin model). 4. Examination of the phosphatase activity of these model systems using 31P NMR. 5. Provision of bridging carboxylate groups, to give the first unsymmetrical models for Hr, in order to study ligand binding processes. 6. Synthesis of new ligand systems leading to stable diiron(II) models for deoxy-Hr and reversible O2 binding. 7. Incorporation of phenol onto the ligand to study Fe(III)OFe(IV)..phenol interactions, relevant to ribonucleotide reductase where a tyrosine cation radical is stabilized by proximity to the FeOFe group.